This invention relates to a method of removing aerosols by the radiation effect, in which an aerosol-containing ambient gas is exposed to a radiation so that the gas is ionized to pairs of positive and negative ions and in which either positive or negative ions are allowed to collide with aerosol particles by Brownian motion so that the aerosol particles are charged to a single polarity and in which the thus charged aerosol particles are removed by a suitable technique such as deposition on the surface of a substance by image force or trapping on an electrode of the opposite polarity or filter by Image force.
It is well known that when an ambient gas is exposed to a radiation, electrons are knocked out of the molecules of the gas to generate ionic pairs, with the positive ion being provided by the molecular ion and the negative ton by electrons.
The present inventors conducted an experiment in which an aerosol was irradiated with gamma rays to generate a charged aerosol, which was then passed through a screen electrode shielded from radiations. As it turned out, the percent deposition of particles on the electrodes with a size of 0.4 .mu.m and more was better than the percent trapping by a HEPA (high-efficiency particulate air) filter. It was also found that when the charged aerosol was passed through both the porous electrodes and the HEPA filter, the electrostatic force of the charged particles contributed to improve the trapping efficiency by approximately 10,000 times as high as the inherent efficiency of the filter. The HEPA filter is composed of extremely fine glass fibers and capable of trapping particles not larger than 10 .mu.m with a very high efficiency of at least 99.97%. The term "image force" as used herein means an electrostatic force that is provided by either positively or negatively charged aerosol particles approaching an uncharged substance in such a way that the surface of the latter is polarized to the opposite polarity, thereby trapping the aerosol particles on that surface.
Since the irradiation with gamma-rays and electron beams finds a wide range of applicability, the performance data set forth in the preceeding paragraph have potentially a far-reaching effect from the viewpoint of removing aerosols on an industrial scale. For example, the technique under consideration is applicable not only in cleanrooms used in the semiconductor and other electronic industries and biotechnological industries; it is also applicable to the treatment of aerosols of high concentration and, hence, it can be used in the treatment of flue gases from thermal power plants and general engineering practices. Further, experiments have shown that in the nuclear field, the technique holds promise for use not only in the incineration of burnable wastes of low radioactivity level but also in incinerators of medium-level wastes which have heretofore been difficult to incinerate.
(a) Producing charged aerosols by corona discharge PA1 (b) Production of unipolar aerosols by alpha-particle charging to a single polarity PA1 (c) Charged filters (for trapping by the electrostatic effect)
Charged aerosols are typically produced by corona discharge. In corona discharge, an intense dc electric field is applied between a plate electrode and a linear electrode so that discharge is caused to take place within the small region around tile linear electrode, thereby ionizing the ambient gas. If a negative potential is applied to the linear electrode, the positive ions will move toward the linear electrode whereas the electrons will move toward the plate (cylindrical) electrode.
The strength of potential decreases sharply with the increasing distance from the linear electrode and, hence, the velocity of electrons decreases as they come closer to the plate electrode and Brownian motion causes the electrons to collide with the particles of the ambient gas, thereby forming negative ions. The region of discharge is small and, therefore, if tile aerosol to be treated is introduced into an electric field, the aerosol particles will collide with negative ions to be charged negatively. For successful performance, corona discharge needs a strong electric field and involves difficulty in constructing large equipment. Therefore, the corona discharge technique is essentially unsuitable for large-scale treatment of aerosols.
A study has been reported on the production of monopolar aerosols using alpha particles. In this method, an electrode is placed both on the bottom and in the upper part of a small vessel about 10 cm high and a weak dc electric field is applied between the two electrodes. An alpha-particle source is mounted on the electrode on the bottom of the vessel. Upon irradiation with alpha particles, ionization occurs to produce ionic pairs consisting of positive and negative ions and, depending on the polarities of the electrodes, either positive or negative ions will move outside the range of alpha particles (which is about 4 cm) and mix with the aerosol flowing in the upper part of the vessel, whereby the aerosol particles are charged to a single polarity. In the reported study, the monopolar aerosol was passed through a filter to determine the effect of image force empirically and the image force of charged particles with respect to the filter was analyzed theoretically.
The use of alpha particles for charging to a single polarity depends on diffusive charging for electrification and hence is an effective method for charging submicron particles. "Diffusive charging" is a phenomenon in which aerosol particles are charged by the collision between those particles and the monopolar ions due to Brownian motion. A problem with the approach of the alpha charging method is that the short range of alpha particles introduces difficulty in constructing large equipment, thereby making the procedure unsuitable for large-scale treatment of aerosols.
For efficient trapping of submicron particles with filters, one may reduce the size of fibers in the filter medium but then the pressure loss that occurs in the filter will increase so much as to reduce the throughput to an impractical low level. Under the circumstances, studies have been conducted to trap aerosols on filters by making use of inductive force and charged filters suittable for use to that end have been commercialized.
The term "inductive force" as used herein means a force that acts on uncharged aerosol particles approaching a substance charged to a single polarity in such a way that the particles are polarized to the opposite polarity, whereby the latter are trapped on the surface of the charged substance. Charged filters have large fiber sizes, so if aerosol particles deposit on the filter, the charges in it are neutralized to cause a marked drop in trapping efficiency, which has been a serious problem with the prior art.